The present invention is directed to methods of making lyocell fibers. In particular, the process includes reducing the average degree of polymerization of a pulp to about 200 to 1100 without substantially reducing the hemicellulose content of the pulp, and reducing the copper numbers to less than 2.0.
Cellulose is a polymer of D-glucose and is a structural component of plant cell walls. Cellulose is especially abundant in tree trunks from which it is extracted, converted into pulp, and thereafter utilized to manufacture a variety of products. Rayon is the name given to a fibrous form of regenerated cellulose that is extensively used in the textile industry to manufacture articles of clothing. For over a century strong fibers of rayon have been produced by the viscose and cuprammonium processes. The latter process was first patented in 1890 and the viscose process two years later. In the viscose process cellulose is first steeped in a mercerizing strength caustic soda solution to form an alkali cellulose. This is reacted with carbon disulfide to form cellulose xanthate which is then dissolved in dilute caustic soda solution. After filtration and deaeration the xanthate solution is extruded from submerged spinnerets into a regenerating bath of sulfuric acid, sodium sulfate, zinc sulfate, and glucose to form continuous filaments. The resulting so-called viscose rayon is presently used in textiles and was formerly widely used for reinforcing rubber articles such as tires and drive belts.
Cellulose is also soluble in a solution of ammoniacal copper oxide. This property forms the basis for production of cuprammonium rayon. The cellulose solution is forced through submerged spinnerets into a solution of 5% caustic soda or dilute sulfuric acid to form the fibers, which are then decoppered and washed. Cuprammonium rayon is available in fibers of very low deniers and is used almost exclusively in textiles.
The foregoing processes for preparing rayon both require that the cellulose be chemically derivatized or complexed in order to render it soluble and therefore capable of being spun into fibers. In the viscose process, the cellulose is derivatized, while in the cuprammonium rayon process, the cellulose is complexed. In either process, the derivatized or complexed cellulose must be regenerated and the reagents that were used to solubilize it must be removed. The derivatization and regeneration steps in the production of rayon significantly add to the cost of this form of cellulose fiber. Consequently, in recent years attempts have been made to identify solvents that are capable of dissolving underivatized cellulose to form a dope of underivatized cellulose from which fibers can be spun.
One class of organic solvents useful for dissolving cellulose are the amine-N oxides, in particular the tertiary amine-N oxides. For example, Graenacher, in U.S. Pat. No. 2,179,181, discloses a group of amine oxide materials suitable as solvents. Johnson, in U.S. Pat. No. 3,447,939, describes the use of anhydrous N-methylmorpholine-N-oxide (NMMO) and other amine N-oxides as solvents for cellulose and many other natural and synthetic polymers. Franks et al., in U.S. Pat. Nos. 4,145,532 and 4,196,282, deal with the difficulties of dissolving cellulose in amine oxide solvents and of achieving higher concentrations of cellulose.
Lyocell is an accepted generic term for a fiber composed of cellulose precipitated from an organic solution in which no substitution of hydroxyl groups takes place and no chemical intermediates are formed. Several manufacturers presently produce lyocell fibers, principally for use in the textile industry. For example, Acordis, Ltd. presently manufactures and sells a lyocell fiber called Tencel(copyright) fiber.
Currently available lyocell fibers suffer from one or more disadvantages. One disadvantage of some lyocell fibers made presently is a function of their geometry which tends to be quite uniform, generally circular or oval in cross section and lacking crimp as spun. In addition, many current lyocell fibers have relatively smooth, glossy surfaces. These characteristics make such fibers less than ideal as staple fibers in woven articles since it is difficult to achieve uniform separation in the carding process and can result in non-uniform blending and uneven yarn.
In addition, fibers having a continuously uniform cross section and glossy surface produce yarns tending to have an unnatural, xe2x80x9cplasticxe2x80x9d appearance. In part to correct the problems associated with straight fibers, man-made staple fibers are almost always crimped in a secondary process prior to being chopped to length. Examples of crimping can be seen in U.S. Pat. Nos. 5,591,388 or 5,601,765 to Sellars et al. where a fiber tow is compressed in a stuffer box and heated with dry steam. Inclusion of a crimping step increases the cost of producing lyocell fibers.
Another widely-recognized problem associated with prior art lyocell fibers is fibrillation of the fibers under conditions of wet abrasion, such as might result during laundering. Fibrillation is defined as the splitting of the surface portion of a single fiber into smaller microfibers or fibrils. The splitting occurs as a result of wet abrasion caused by attrition of fiber against fiber or by rubbing fibers against a hard surface. Depending on the conditions of abrasion, most or many of the microfibers or fibrils will remain attached at one end to the mother fiber. The microfibers or fibrils are so fine that they become almost transparent, giving a white, frosty appearance to a finished fabric. In cases of more extreme fibrillation, the microfibers or fibrils become entangled, giving the appearance and feel of pilling, i.e., entanglement of fibrils into small, relatively dense balls.
Fibrillation of lyocell fibers is believed to be caused by the high degree of molecular orientation and apparent poor lateral cohesion of microfibers or fibrils within the fibers. There is extensive technical and patent literature discussing the problem and proposed solutions. As examples, reference can be made to papers by Mortimer, S. A. and A. A. Pxc3xa9guy, Journal of Applied Polymer Science, 60:305-316 (1996) and Nicholai, M., A. Nechwatal, and K. P. Mieck, Textile Research Journal 66(9):575-580 (1996). The first authors attempt to deal with the problem by modifying the temperature, relative humidity, gap length, and residence time in the air gap zone between extrusion and dissolution. Nicholai et al. suggest crosslinking the fiber but note that xe2x80x9cat the moment, technical implementation [of the various proposals] does not seem to be likelyxe2x80x9d. A sampling of related United States patents includes those to Taylor, U.S. Pat. Nos. 5,403,530, 5,520,869, 5,580,354, and 5,580,356; Urben, U.S. Pat. No. 5,562,739; and Weigel et al. U.S. Pat. No. 5,618,483. These patents in part relate to treatment of the fibers with reactive materials to induce surface modification or crosslinking. Enzymatic treatment of yarns or fabrics is currently the preferred way of reducing problems caused by fibrillation; however, all of the treatments noted have disadvantages, including increased production costs.
Additionally, it is believed that currently available lyocell fibers are produced from high quality wood pulps that have been extensively processed to remove non-cellulose components, especially hemicellulose. These highly processed pulps are referred to as dissolving grade or high alpha (or high xcex1) pulps, where the term alpha (or xcex1) refers to the percentage of cellulose. Thus, a high alpha pulp contains a high percentage of cellulose, and a correspondingly low percentage of other components, especially hemicellulose. The processing required to generate a high alpha pulp significantly adds to the cost of lyocell fibers and products manufactured therefrom.
For example, in the Kraft process a mixture of sodium sulphide and sodium hydroxide is used to pulp the wood. Since conventional Kraft processes stabilize residual hemicelluloses against further alkaline attack, it is not possible to obtain acceptable quality dissolving pulps, i.e., high alpha pulps, through subsequent treatment in the bleach plant. In order to prepare dissolving type pulps by the Kraft process, it is necessary to give the chips an acidic pretreatment before the alkaline pulping stage. A significant amount of material, on the order of 10% of the original wood substance, is solubilized in this acid phase pretreatment. Under the prehydrolysis conditions, the cellulose is largely resistant to attack, but the residual hemicelluloses are degraded to a much shorter chain length and can therefore be removed to a large extent in the subsequent Kraft cook by a variety of hemicellulose hydrolysis reactions or by dissolution. Primary delignification also occurs during the Kraft cook.
The prehydrolysis stage normally involves treatment of wood at elevated temperature (150-180xc2x0 C.) with dilute mineral acid (sulfuric or aqueous sulfur dioxide) or with water alone requiring times up to 2 hours at the lower temperature. In the latter case, liberated acetic acid from certain of the naturally occurring polysaccharides (predominantly the mannans in softwoods and the xylan in hardwoods) lowers the pH to a range of 3 to 4.
While the prehydrolysis can be carried out in a continuous digester, typically the prehydrolysis is carried out in a batch digester. As pulp mills become larger and the demand for dissolving grade pulp increases, more batch digesters will be needed to provide prehydrolyzed wood. The capital cost of installing such digesters and the costs of operating them will contribute to the cost of dissolving grade pulps. Further, prehydrolysis results in the removal of a large amount of wood matter and so pulping processes that incorporate a prehydrolysis step are low yield processes.
Moreover, a relatively low copper number is a desirable property of a pulp that is to be used to make lyocell fibers because it is generally believed that a high copper number causes cellulose degradation during and after dissolution in an amine oxide solvent. The copper number is an empirical test used to measure the reducing value of cellulose. Further, a low transition metal content is a desirable property of a pulp that is to be used to make lyocell fibers because, for example, transition metals accelerate the degradation of cellulose and NMMO in the lyocell process.
Thus, there is a need for relatively inexpensive, low alpha pulps that can be used to make lyocell fibers, for a process for making the foregoing low alpha pulps, and for lyocell fibers from the foregoing low alpha pulp. Preferably the desired low alpha pulps will have a low copper number, a low lignin content and a low transition metal content. Preferably it will be possible to use the foregoing low alpha pulps to make lyocell fibers having a decreased tendency toward fibrillation and a more natural appearance compared to presently available lyocell fibers.
As used herein, the terms xe2x80x9ccomposition(s) of the present inventionxe2x80x9d, or xe2x80x9ccomposition(s) useful for making lyocell fibersxe2x80x9d, or xe2x80x9ccomposition(s), useful for making lyocell fibers,xe2x80x9d or xe2x80x9ctreated pulpxe2x80x9d or xe2x80x9ctreated Kraft pulpxe2x80x9d refer to pulp, containing cellulose and hemicellulose, that has been treated in order to reduce the average degree of polymerization (D.P.) of the cellulose without substantially reducing the hemicellulose content of the pulp. The compositions of the present invention preferably possess additional properties as described herein.
Accordingly, the present invention provides compositions useful for making lyocell fibers, or other molded bodies such as films, having a high hemicellulose content, a low lignin content and including cellulose that has a low average D.P. Preferably, the cellulose and hemicellulose are derived from wood, more preferably from softwood. Preferably, the compositions of the present invention have a low copper number, a low transition metal content, a low fines content and a high freeness. Compositions of the present invention may be in a form that is adapted for storage or transportation, such as a sheet, roll or bale. Compositions of the present invention may be mixed with other components or additives to form pulp useful for making lyocell molded bodies, such as fiber or films. Further, the present invention provides processes for making compositions, useful for making lyocell fibers, having a high hemicellulose content, a low lignin content and including cellulose that has a low average D.P. The present invention also provides lyocell fibers containing cellulose having a low average D.P., a high proportion of hemicellulose and a low lignin content. The lyocell fibers of the present invention also preferably possess a low copper number and a low transition metal content. In one embodiment, preferred lyocell fibers of the present invention possess a non-lustrous surface and a natural crimp that confers on them the appearance of natural fibers. Further, the preferred lyocell fibers of the present invention have enhanced dye-binding properties and a reduced tendency to fibrillate.
Compositions of the present invention can be made from any suitable source of cellulose and hemicellulose but are preferably made from a chemical wood pulp, more preferably from a Kraft softwood pulp, most preferably from a bleached, Kraft softwood pulp, which is treated to reduce the average D.P. of the cellulose without substantially reducing the hemicellulose content. Compositions of the present invention include at least 7% by weight hemicellulose, preferably from 7% by weight to about 30% by weight hemicellulose, more preferably from 7% by weight to about 20% by weight hemicellulose, most preferably from about 10% by weight to about 17% by weight hemicellulose, and cellulose having an average D.P. of from about 200 to about 1100, preferably from about 300 to about 1100, and more preferably from about 400 to about 700. A presently preferred composition of the present invention has a hemicellulose content of from about 10% by weight to about 17% by weight, and contains cellulose having an average D.P. of from about 400 to about 700. Hemicellulose content is measured by a proprietary Weyerhaeuser sugar content assay. Further, compositions of the present invention have a kappa number of less than 2, preferably less than 1. Most preferably compositions of the present invention contain no detectable lignin. Lignin content is measured using TAPPI Test T236om85.
Compositions of the present invention preferably have a unimodal distribution of cellulose D.P. values wherein the individual D.P. values are approximately normally distributed around a single, modal D.P. value, i.e., the modal D.P. value being the D.P. value that occurs most frequently within the distribution. The distribution of cellulose D.P. values may, however, be multimodal i.e., a distribution of cellulose D.P. values that has several relative maxima. A multimodal, treated pulp of the present invention might be formed, for example, by mixing two or more unimodal, treated pulps of the present invention that each have a different modal D.P. value. The distribution of cellulose D.P. values is determined by means of proprietary assays performed by Thuringisches Institut fur Textil-und Kunstoff Forschunge. V., Breitscheidstr. 97, D-07407 Rudolstadt, Germany. Preferably the compositions of the present invention have a reduced fines content, a freeness that is comparable to untreated pulp, and a length-weighted percentage of fibers, of length less than 0.2 mm, of less than about 4%.
Additionally, compositions of the present invention preferably have a copper number of less than about 2.0, more preferably less than about 1.1, most preferably less than about 0.7 as measured by Weyerhaeuser Test Method PPD3. Further, compositions of the present invention preferably have a carbonyl content of less than about 120 xcexcmol/g and a carboxyl content of less than about 120 xcexcmol/g. The carboxyl and carbonyl group content are measured by means of proprietary assays performed by Thuringisches Institut fur Textil-und Kunstoff Forschunge. V., Breitscheidstr. 97, D-07407 Rudolstadt, Germany.
Compositions of the present invention also preferably possess a low transition metal content. Preferably, the total transition metal content of the compositions of the present invention is less than 20 ppm, more preferably less than 5 ppm, as measured by Weyerhaeuser Test Number AM5-PULP-1/6010. The term xe2x80x9ctotal transition metal contentxe2x80x9d refers to the combined amounts, measured in units of parts per million (ppm), of nickel, chromium, manganese, iron and copper. Preferably the iron content of the compositions of the present invention is less than 4 ppm, more preferably less than 2 ppm, as measured by Weyerhaeuser Test AM5-PULP-1/6010, and the copper content of the compositions of the present invention is preferably less than 1.0 ppm, more preferably less than 0.5 ppm, as measured by Weyerhaeuser Test AM5-PULP-1/6010.
Compositions of the present invention are readily soluble in amine oxides, including tertiary amine oxides such as NMMO. Other preferred solvents that can be mixed with NMMO, or another tertiary amine solvent, include dimethylsulfoxide (D.M.S.O.), dimethylacetamide (D.M.A.C.), dimethylformamide (D.M.F.) and caprolactan derivatives. Preferably, compositions of the present invention fully dissolve in NMMO in less than about 70 minutes, preferably less than about 20 minutes, utilizing the dissolution procedure described in Example 6 herein. The term xe2x80x9cfully dissolvexe2x80x9d, when used in this context, means that substantially no undissolved particles are seen when a dope, formed by dissolving compositions of the present invention in NMMO, is viewed under a light microscope at a magnification of 40xc3x97 to 70xc3x97.
The compositions of the present invention may be in a form, such as a sheet, a roll or a bale, that is adapted for convenient and economical storage and/or transportation. In a particularly preferred embodiment, a sheet of a composition of the present invention has a Mullen Burst Index of less than about 2.0 kN/g (kiloNewtons per gram), more preferably less than about 1.5 kN/g, most preferably less than about 1.2 kN/g. The Mullen Burst Index is determined using TAPPI Test Number T-220. Further, in a particularly preferred embodiment a sheet of a composition of the present invention has a Tear Index of less than 14 mNm2/g, more preferably less than 8 mNm2/g, most preferably less than 4 mNm2/g. The Tear Index is determined using TAPPI Test Number T-220.
A first preferred embodiment of the treated pulp of the present invention is a treated Kraft pulp including at least 7% by weight hemicellulose, a copper number less than about 2.0 and cellulose having an average degree of polymerization of from about 200 to about 1100.
A second preferred embodiment of the treated pulp of the present invention is a treated Kraft pulp including at least 7% by weight hemicellulose, a kappa number less than two and cellulose having an average degree of polymerization of from about 200 to about 1100, the individual D.P. values of the cellulose being distributed unimodally.
A third preferred embodiment of the treated pulp of the present invention is a treated Kraft pulp including at least 7% by weight hemicellulose, cellulose having an average degree of polymerization of from about 200 to about 1100, a kappa number less than two and a copper number less than 0.7.
A fourth preferred embodiment of the treated pulp of the present invention is a treated Kraft pulp including at least 7% by weight hemicellulose, cellulose having an average degree of polymerization of from about 200 to about 1100, a kappa number less than two, an iron content less than 4 ppm and a copper content less than 1.0 ppm.
A fifth preferred embodiment of the treated pulp of the present invention is a treated Kraft pulp including at least 7% by weight hemicellulose, cellulose having an average degree of polymerization of less than 1100, and a lignin content of about 0.1 percent by weight.
In another aspect, the present invention provides lyocell fibers including at least about 5% by weight hemicellulose, preferably from about 5% by weight to about 27% by weight hemicellulose, more preferably from about 5% by weight to about 18% by weight hemicellulose, most preferably from about 10% by weight to about 15% by weight hemicellulose, and cellulose having an average D.P. of from about 200 to about 1100, more preferably from about 300 to about 1100, most preferably from about 400 to about 700. Additionally, preferred lyocell fibers of the present invention have a unimodal distribution of cellulose D.P. values, although lyocell fibers of the present invention may also have a multimodal distribution of cellulose D.P. values, i.e., a distribution of cellulose D.P. values that has several relative maxima. Lyocell fibers of the present invention having a multimodal distribution of cellulose D.P. values might be formed, for example, from a mixture of two or more unimodal, treated pulps of the present invention that each have a different modal D.P. value.
Preferred lyocell fibers of the present invention have a copper number of less than about 2.0, more preferably less than about 1.1, most preferably less than about 0.7 as measured by Weyerhaeuser Test Number PPD3. Further, preferred lyocell fibers of the present invention have a carbonyl content of less than about 120 xcexcmol/g and a carboxyl content of less than about 120 xcexcmol/g. The carboxyl and carbonyl group content are measured by means of proprietary assays performed by Thuringisches Institut fur Textil-und Kunstoff Forschunge. V., Breitscheidstr. 97, D-07407 Rudolstadt, Germany. Additionally, preferred lyocell fibers of the present invention have a total transition metal content of less than about 20 ppm, more preferably less than about 5 ppm, as measured by Weyerhaeuser Test Number AM5-PULP-1/6010. The term xe2x80x9ctotal transition metal contentxe2x80x9d refers to the combined amount, expressed in units of parts per million (ppm), of nickel, chromium, manganese, iron and copper. Preferably the iron content of lyocell fibers of the present invention is less than about 4 ppm, more preferably less than about 2 ppm, as measured by Weyerhaeuser Test AM5-PULP-1/6010, and the copper content of lyocell fibers of the present invention is preferably less than about 1 ppm, more preferably less than about 0.5 ppm, as measured by Weyerhaeuser Test AM5-PULP-1/6010. Lyocell fibers of the present invention have a kappa number of less than 2.0, preferably less than 1.0.
In preferred embodiments lyocell fibers of the present invention have a pebbled surface and a non-lustrous appearance. Preferably the reflectance of a wet-formed handsheet made from lyocell fibers of the present invention is less than about 8%, more preferably less than 6%, as measured by TAPPI Test Method T480-om-92.
Additionally, lyocell fibers of the present invention preferably have a natural crimp of irregular amplitude and period that confers a natural appearance on the fibers. Preferably the crimp amplitude is greater than about one fiber diameter and the crimp period is greater than about five fiber diameters. Preferred embodiments of lyocell fibers of the present invention also possess desirable dye-absorptive capacity and resistance to fibrillation. Further, preferred embodiments of the lyocell fibers of the present invention also possess good elongation. Preferably, lyocell fibers of the present invention possess a dry elongation of from about 8% to about 17%, more preferably from about 13% to about 15%. Preferably, lyocell fibers of the present invention possess a wet elongation of from about 13% to about 18%. Elongation is measured by means of proprietary assays performed by Thuringisches Institut fur Textil-und Kunstoff Forschunge. V., Breitscheidstr. 97, D-07407 Rudolstadt, Germany.
A presently preferred lyocell fiber of the present invention includes cellulose from treated Kraft pulp having at least 5% by weight hemicellulose, cellulose having an average D.P. of 200 to 1100 and a kappa number of less than two.
In another aspect, the present invention provides processes for making compositions of the present invention that can, in turn, be formed into lyocell molded bodies, such as fibers or films. In a first embodiment, the present invention provides a process that includes contacting a pulp comprising cellulose and hemicellulose with an amount of a reagent sufficient to reduce the average D.P. of the cellulose to within the range of from about 200 to about 1100, preferably to within the range of from about 300 to about 1100, more preferably to within the range of from about 400 to about 700, without substantially reducing the hemicellulose content. This D.P. reduction treatment occurs after the pulping process and before, during or after the bleaching process, if a bleaching step is utilized. The reagent is preferably at least one member of the group consisting of acid, steam, alkaline chlorine dioxide, the combination of at least one transition metal and a peracid, preferably peracetic acid, and the combination of ferrous sulfate and hydrogen peroxide. Preferably the copper number of the treated pulp is reduced to a value less than about 2.0, more preferably less than about 1.1, most preferably less than about 0.7. The copper number is measured by Weyerhaeuser test PPD3.
Presently the most preferred acid is sulfuric acid. The acid, or combination of acids, is preferably utilized in an amount of from about 0.1% w/w to about 10% w/w in its aqueous solution, and the pulp is contacted with the acid for a period of from about 2 minutes to about 5 hours at a temperature of from about 20xc2x0 C. to about 180xc2x0 C.
When the reagent is steam, the steam is preferably utilized at a temperature of from about 120xc2x0 C. to about 260xc2x0 C., at a pressure of from about 150 psi to about 750 psi, and the pulp is exposed to the steam for a period of from about 0.5 minutes to about 10 minutes. Preferably the steam includes at least one acid. Preferably, the steam includes an amount of acid sufficient to reduce the pH of the steam to a value within the range of from about 1.0 to about 4.5.
When the reagent is a combination of at least one transition metal and peracetic acid, the transition metal(s) is present at a concentration of from about 5 ppm to about 50 ppm, the peracetic acid is present at a concentration of from about 5 mmol per liter to about 200 mmol per liter, and the pulp is contacted with the combination for a period of from about 0.2 hours to about 3 hours at a temperature of from about 40xc2x0 C. to about 100xc2x0 C.
When the reagent is a combination of ferrous sulfate and hydrogen peroxide, the ferrous sulfate is present at a concentration of from about 0.1 M to about 0.6 M, the hydrogen peroxide is present at a concentration of from about 0.1% v/v to about 1.5% v/v, and the pulp is contacted with the combination for a period of from about 10 minutes to about one hour at a pH of from about 3.0 to about 5.0.
Preferably the yield of the first embodiment of a process for making compositions of the present invention is greater than about 95%, more preferably greater than about 98%. The process yield is the dry weight of the treated pulp produced by the process divided by the dry weight of the starting material pulp, the resulting fraction being multiplied by one hundred and expressed as a percentage.
In another aspect of the present invention a process for making lyocell fibers includes the steps of (a) contacting a pulp including cellulose and hemicellulose with an amount of a reagent sufficient to reduce the average degree of polymerization of the cellulose to the range of from about 200 to about 1100, preferably to the range of from about 300 to about 1100, without substantially reducing the hemicellulose content; and (b) forming fibers from the pulp treated in accordance with step (a). The copper number of the treated pulp is preferably reduced to a value less than 2.0 prior to fiber formation. In accordance with this aspect of the present invention, the lyocell fibers are preferably formed by a process selected from the group consisting of melt blowing, centrifugal spinning, spun bonding and a dry jet/wet process.